Since the start of the US blockade on Monday, 15 vessels have crossed the Strait of Hormuz, nine of which have links to Iran, BBC Verify analysis of ship-tracking data suggests.

US Central Command (Centcom) has said its blockade applies to all ships entering and exiting Iranian ports.

In an update on Tuesday, Centcom said in the first 36 hours no vessels made it past the blockade and six obeyed instructions to turn around.

Ship-tracking data suggests six of the ships which passed through the strait had previously visited an Iranian port. We have identified two more that left the vicinity of an Iranian port far to the east of the strait.

President Donald Trump has said the US blockade is in response to Iran "knowingly failing" to reopen the strait.

BBC Verify has not seen evidence from the available tracking data of any tankers carrying full loads of oil or gas heading out from the Gulf into open sea during this period, whether linked to Iran or not.

Tracking vessels relies on them transmitting their locations accurately.  However, some ships may be turning their trackers off or be transmitting a false position, a practice known as "spoofing".

Experts have told BBC Verify this is common for sanctioned tankers linked to Iran.

We've not been able to verify the US claim about the number of ships that have been turned round, but tracking data indicates that at least one vessel which passed through the strait has reversed course.

The US-sanctioned tanker Rich Starry sailed east from Sharjah in the UAE through the strait but turned round and crossed back into the Gulf on Wednesday.

In addition, two Iranian-flagged ships have been tracked leaving the vicinity of Iran's Chahbahar port, east of the Strait of Hormuz on Iran's southern coast, after the start of the blockade.

The Strait of Hormuz has become a focal point of the US-Israel war with Iran after Tehran effectively choked off one of the world's most important shipping lanes.

An average of 138 ships passed through the strait each day before the conflict started on 28 February, according to the Joint Maritime Information Centre.

Trump has warned the US Navy will "blow to hell" any Iranians that attack them and will take action against any ship found to be paying transit tolls to Iran.

The disruption to shipping since the conflict began six weeks ago has sent shock waves across the global economy, destabilising energy prices and exposing just how reliant international supply chains are on the channel that connects the Gulf with the Indian Ocean.

Jakob Larsen from the Baltic and International Maritime Council, which represents shipowners, told BBC Verify that he is concerned about the "risk of further escalation to involve direct attacks on ships" with the introduction of the US blockade.

If crossings for ships not linked to Iran do resume at a greater pace, experts say stranded tankers that are fully loaded with cargo will be the priority.

"You've had nearly 800 ships stuck in there for several weeks. Most of them are now loaded with cargo so the priority is going to be to get them out," said Richard Meade, editor-in-chief of Lloyd's List

BBC Verify's analysis shows ships that have passed the strait have taken a northerly route close to Iran's coastline and within its territorial waters.

Prior to the conflict, vessels usually took a more southerly route through the middle of the waterway.

Another uncertainty is the possibility of sea mines, says Thomas Kazakos, secretary general of the International Chamber of Shipping.

"We need to make sure that we have clear confirmation that the safety of navigation for the ships and the seafarers are being agreed," he told BBC Verify.

Iran's Islamic Revolutionary Guard Corps (IRGC) Navy published what it states are the permitted routes to be followed when passing through the strait "to avoid possible collision with sea mines".

It has marked a region in the middle of the channel as a "dangerous area" to be avoided.

Centcom has previously announced that two destroyers - the USS Frank E Petersen and USS Michael Murphy - are in the area as part of a mission to clear "sea mines previously laid by Iran's Islamic Revolutionary Guards Corps".

Additional reporting by Kayleen Devlin and Alex Murray

15 April: This piece is being updated to reflect the latest number of vessels crossing the strait since the ceasefire.

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